Brief Description: Countersinking Spar for Tank Attachment
The fuel tank attach platenuts will accept #8 flathead screws. A countersink must be made opposite each platenut into the spar flange. That countersink accepts the dimpled tank skin and a #8 screw goes through the whole mess. The tank and tank skin come later but this must be done now. Since countersinking removes material (and the anodizing on the spar), each hole must be treated with alodine (or at least a protective primer). Image 3 shows an improvised depth gauge to measure the depth of the countersunk holes in the spar. It is a #8 screw with the threads ground off so that it easily fit through the platenut and the top of the screw can nest in the countersink. I epoxied the screw head to a piece of aluminum gutter nail to make it easy to handle. Keeping in mind that the screwhead fits in the skin dimple and not the countersink in the spar, it is used to gauge comparitive depth and not the diameter of the countersink. The countersink must accept the underside of the dimpled skin.